nir: Turn imov/fmov of undef into undef
nir: Turn imov/fmov of undef into undef

Reverting the previous attempt at this a5502a721fd30fd resulted in
the following Vulkan test failing.

dEQP-VK.glsl.return.return_in_dynamic_loop_dynamic_vertex

This time we use the num_components from the alu dest rather than
num_inputs to the op to determine the size of the undef.

Reviewed-by: Kenneth Graunke <bd2562f754ec92342f93f61c25d731e290a2ffa8@whitecape.org>
Reviewed-by: Jason Ekstrand <68c46a606457643eab92053c1c05574abb26f861@jlekstrand.net>
Cc: "13.0" <59f39c0db42d4479a46b02d4d2bc11120e37bb44@lists.freedesktop.org>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=99100

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler